Digital micro-mirror device(DMD) is becoming a widely used display apparatus owing to its high contrast, high speed and high resolution. One may refer to U.S. Pat. No. 4,680,579 for details of a DMD apparatus. A DMD consists of a matrix of micro-mirrors contained in a common plane. Each micro-mirror is of miniature size: as small as a few microns. The micro-mirrors are individually adjustable between a first and second position by a voltage applied to each micro-mirror pixel. The respective positions may be identified as "on" and "off". If a particular micro-mirror of a DMD is in the "on" position, an incident illumination light is reflected by this micro-mirror and is delivered to a viewing device. Such reflected illumination light is called "useful" illumination light. If a micro-mirror is in the "off" position, the reflected illumination light is spatially displaced with respect to the light beam reflected by it in the "on" position, and is not received by the viewing device. Illumination light which is reflected by a micro-mirror in "off" position is called "useless" illumination light.
A DMD apparatus can be used in an image projection system as well as a holographic system. In a holographic system, the DMD apparatus is used to encode a signal beam. One may refer to a concurrently filed U.S. Pat. application titled "Mirror-Based Off-Axis Optical System for Holographic Storage" for an example of this application.
In a particular application, a DMD apparatus is often used in conjunction with a light source, an optical system which can couple the illumination light from the light source to the DMD, and can subsequently couple the reflected illumination light from the DMD apparatus to a viewing device. When a DMD is used in an image projection system, the light source is typically a broad band white light source. When a DMD apparatus is used in a holographic storage system, the light source is typically a mono-chromatic coherent laser. The optical system described above is often called an DMD coupler.
One important characteristic of a DMD system is that the miniature sized micro-mirrors cause diffraction of the illumination light. FIG. 1 shows an illumination beam reflected from a micro-mirror pixel of a DMD, forming a diffraction limited reflected beam. In FIG. 1, a DMD 102 comprises a matrix of micro-mirrors 104. The size of each micro-mirror 104, for example, can be as small as 17 micron square. Diffraction effect occurs when an incident light beam 106 is reflected by this micro-mirror pixel 104. The reflected beam 110 has a beam profile which comprises a primary peak 112, a first order diffraction 114, a second order diffraction 116, and higher orders of diffraction which are not shown in the figure. The diffraction cone angle is approximately 1.9 degrees when the first order diffraction is included, and approximately 3.8 degrees when two orders of diffraction are included. When a DMD apparatus is used to encode a signal beam in a holographic system, this diffraction effect reduces the resolution of a holographic storage system, and consequently reduces the capacity of the holographic storage system.
A DMD coupler which couples the illumination light to the DMD, and couples the illumination light reflected by micro-mirrors out of the DMD, is an important part of a DMD system. Examples of such DMD couplers are taught in U.S. Pat. No. 5,552,922, and U.S. Pat. No. 5,420,655. FIG. 2 is a schematic drawing illustrating a DMD coupler taught in the prior art. Illumination beam 200 is totally reflected at an air gap 214 between a prism 212 and a prism 216. The refracted illumination beam 206 is reflected by a micro-mirror 104 of a DMD 102. The reflected beam 208 is incident on the interface of prism 212 and the air gap 214 at an angle which is smaller than the critical angle. The reflected beam 206 passes the air gap 214 and exits the DMD coupler, forming an output beam 210. Light beam 208 is diffraction limited due to the diffraction at the micro-mirrors. Refraction of this diffraction limited beam 208 by the air gap 214 leads to a decrease of the optical quality of the system. Such reduction of the optical quality is detrimental to a holographic storage application. For example, an air gap 214 having a thickness of 50 .mu.m will reduce the Strehl Ratio from 1 to 0.985 when one order of diffraction is included in the holographic storage medium. A further drop of Strehl Ratio from 1 to 0.772 occurs when 2 orders of diffraction are included. A holographic data storage system often requires a Strehl Ratio of more than 0.90 to ensure data fidelity in a system which uses a DMD encoded signal beam. In the first case, when one diffraction order is included in the storage medium, the prism coupler alone consumes 15% of the overall optical system's tolerance budget. In the second case, when two diffraction orders are included, the prism coupler alone would cause the optical system to fail to meet tolerance specifications.
Furthermore, ghost images may form due to multiple reflection of the light beam 208 in the air gap, since beam 208 is not incident on the air-prism interface 215 at the Brewster's angle.
Thus, a DMD coupler that preserves the optical quality of the "useful" illumination beam is needed.